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Re: [lammps-users] Does LAMMPS use Bounding volume for rigid body collision detection in the broad phase?
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Re: [lammps-users] Does LAMMPS use Bounding volume for rigid body collision detection in the broad phase?

From: KeepFIT <debianfans@...24...>
Date: Thu, 12 Oct 2017 01:48:30 +0200

Dear Alex,

thank for reply.  Suppose rigid body 1 has n1 spheres and rigid body 2 has n2. Do we really need to evaluate the distance of each sphere in body 1 to each sphere in body 2? This operation is an O(n1 x n2) complexity.


As shown in the above picture, Bounding spheres (A, B) that tightly encompass composing spheres of a rigid body are constructed.

(1) If A and B do not intersect, then we can skip the O(n1 x n2) pairwise tests.
(2)  If A and B do intersect, then rigid body 1 and 2 might be colliding, thus the  O(n1 x n2) pairwise tests are needed to calculate the contact force.

I checked the fix_rigid source codes, it seems that in Lammps the step (1) is ignored?

My understanding is that the neighbor list of a sphere in rigid body only contains spheres in other rigid bodies, hence the number of spheres in the list is quite low, as there are more space between angular bodies than that of spheres assembly. The resultant contact force acting on a rigid body is the sum of contact forces on each composing spheres, no need to do (1) and (2) as mentioned above.


On 11.10.2017 23:51, Axel Kohlmeyer wrote:

On Wed, Oct 11, 2017 at 4:36 PM, KeepFIT <debianfans@...24...> wrote:
Dear Lammper users,

I was wondering, whether  the bounding volume technique is used for finite size rigid bodies:

atom_style hybrid sphere molecular

For a pair of rigid bodies 1, 2, if they are far away from each other, it is not necessary to check the distance between spheres in each rigid body. Instead, using two spheres named A, B that encompass these 2 bodies, if A does not intersects B, we can say body 1 is definitely not colliding with body 2. If they do, then we can say they might insect, and detailed contacts between rigid body 1 and 2 need to be calculated.

​interactions in LAMMPS are not computed per rigid body, but per constituent particles. for the interactions between the constituent particle​ LAMMPS uses verlet lists.
so your concern does not apply.





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Dr. Axel Kohlmeyer  akohlmey@...24...
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.